CRETACEOUS TO PALEOGENE TECTONIC EVOLUTION OF THE WESTERN ALASKA RANGE: NEW INSIGHTS FROM SHRIMP U-PB GEOCHRONOLOGY

Exposures in the western Alaska Range contain abundant igneous rocks that were emplaced into or erupted onto Paleozoic to early Mesozoic carbonate and siliciclastic strata of the Farewell terrane and Mesozoic to early Cenozoic turbiditic strata of the Kahiltna basin. Precise U-Pb age determinations are sparse, thus complicating correlation and interpretation of these igneous units both locally and within a regional tectonic and metallogenic framework. New U-Pb ages for zircon and titanite were determined using the sensitive high resolution ion microprobe for six igneous rock suites exposed across the range. Granitoid plutons on the northern and southern shore of Lake Chakachamna crystallized at ca. 98 Ma and 86–85 Ma, respectively, and both plutons are inferred to intrude the southern Kahiltna basin. The ca. 86–85 Ma plutonic rocks are undeformed and have U-Pb titanite ages of ca. 77 Ma, interpreted to represent either slow cooling or a thermal metamorphic overprint. The ca. 98 Ma granitic rocks are foliated to mylonitic, are cut by a ca. 59 Ma mafic dike, and yielded titanite ages of ca. 60–57 Ma. The presence of mylonitic fabric in the dike leads us to interpret the titanite as recording metamorphism associated with deformation along a newly discovered east–northeast-striking, subvertical ductile shear zone with dextral-oblique kinematics. Preliminary mapping indicates that mylonitic fabrics extend approximately 30 km along strike and 1 km across strike. Ca. 62–60 Ma granite and gabbro intruding the Kahiltna succession approximately 7 km to the north are undeformed. Ca. 67–59 Ma andesitic to rhyolitic volcanic rocks erupted onto the Kahiltna succession to the east and northeast are tilted and faulted but are otherwise not strongly deformed. In Farewell terrane exposures to the northwest, new Paleogene igneous ages include the ca. 38 Ma Terra Cotta volcanic field, comprising dacitic to andesitic flows, tuffs, and lahar deposits, and the ca. 32–31 Ma Windy Fork peralkaline granite. These ages, together with previously published K-Ar, Ar-Ar, and U-Pb ages, provide a more complete view of the Cretaceous to Paleogene magmatic record of the western Alaska Range. This area represents a key spatial and tectonic link between regions with more substantial proven and prospective mineral resources to the northeast and southwest.